Electrostatic separation of round and nonround particles



Nov. 11, 1969 R. w. MADRID 3,477,568

ELECTROSTATIC SEPARATION OF ROUND AND NONROUND PARTICLES Filed Nov. 1,1966 2 Sheets-Sheet 1 FIG. 1

INVENTOR. @PERfi W. MADRID BY 5 ATTORNEYS Nov. 11, 1969 R. w. MADRID3,477,568

ELECTROSTATIC SEPARATION OF ROUND AND NONROUND PARTICLES Filed Nov. 1,1966 I 2 Sheets-Sheet 2 INVENTOR. ROBERT w. MADRID agma C. p.21;

A T TORNEYS United States Patent 6 3,477,568 ELECTROSTATIC SEPARATION OFROUND AND NONROUND PARTICLES Robert W. Madrid, Macedon, N.Y., assignorto Xerox Corporation, Rochester, N.Y., a corporation of New York FiledNov. 1, 1966, Ser. No. 591,174 Int. Cl. B03]: 7/08, 7/04 US. Cl. 209-12716 Claims ABSTRACT OF THE DISCLOSURE This invention relates in generalto a sorting system and more particularly to a novel system forseparating spherical particles from nonspherical or otherwise imperfect,irregular particles including fine chips, filaments, powder and dust.

It is often desirable and necessary in certain commercial applicationsto separate, sort, or refine a batch of unrefined particles as to shapeand more specifically to separate spherical particles, hereafter calledrounds, in the batch from the nonrounds which may include irregularrounds, dumbbells i.e. two rounds fused together, aggregates of two ormore rounds, angular shaped particles and fine chips, filaments, powderand dust. One such application is in the manufacture of developermaterials in the process of xerography.

In the process of xerography, for example, as disclosed in CarlsonPatent No. 2,297,691, issued Oct. 6, 1942, a xerographic platecomprising a layer of photoconductive insulating material on aconductive backing is given a uniform electric charge over its surfaceand is then exposed to the subject matter to be reproduced, usually byconventional projection techniques. This exposure discharges the plateareas in accordance with the radiation intensity that reaches them andthereby creates an electrostatic latent image on or in thephotoconductive layer. Development of the latent image is effected withan electrostatically charge finely divided material, such as anelectroscopic powder, that is brought into surface contact with thephotoconductive layer and is held thereon electrostatically in a paterncorresponding to the electrostatic latent image. Thereafter, thedeveloped xerographic powder image is usually transferred to a transfersupport surface for example paper to which it may be afiixed by anysuitable means.

The cascade method of latent image development has found extensivecommercial acceptance and generally consists of gravitationally flowingacross the xerographic plate developer material consisting of a twocomponent material consisting of an electroscopic marking powder termedtoner and a granular material called carrier which consists preferablyof spherical particles devoid of nonrounds including fines which mayfreely reside in the carrier mixture or adhere tenaciously to thesurface of individual carrier beads.

Illustrative examples of such two component developer materials aredisclosed in Walkup et al. Patent 2,638,416, Walkup Patent 2,618,551,Wise Patent 2,618,552 and Copley Patent 2,659,670. Generally carrierparticles are much larger relative to the toner particles, tonerparticles 3,477,568 Patented Nov. 11, 1969 generally having an averageparticle diameter between about 1 and 30 microns whereas the carrierparticles for example may have an average particle diameter from about50 to 700 microns.

As indicated in the aforementioned patents, conventionally the carrierparticles act as vehicles to carry toner to latent image areas and serveto triboelectrically charge toner so that toner may be pulled off of thecarrier particles to image areas but not to background areas. Carrierbeads also pick up toner particles which might tend to ad here touncharged or background areas.

As pointed out in Walkup et al. Patent 2,638,416 it is extremelydesirable to have carriers round or nearly mind as to facilitate theirmovement in gravitating, without sticking, over the xerographic platefor example in the cascade method of xerographic image development.Carrier bead sticking is to be avoided since carrier beads on the plateafter development are carried on the plate through subsequent imagetransfer and plate cleaning steps. Beads on the plate during the imagetransfer step prevent intimate contact of the plate with the transfersupport surface in the region of the bead thus causing incomplete imagetransfer in that region. Also pressure is often required during thistransfer step and this pressure may cause carrier particles to scar,dent or otherwise degrade the relatively delicate surface of thexerographic plate. Pressu re may also misshape or fracture carrierparticles thus rendering them unfit for the further reuse andnecessitating periodic replenishment.

Also in automatic recyclable xerographic copying machines the plate isgenerally cleaned after the transfer step to ready the plate for a newimaging cycle. In the cleaningstep where any remaining toner is removedfrom the plate to ready it for reuse, especially where web belts inskidding contact with the plate are employed as the cleaning means,carrier particles on the plate by being ground against it, may causeserious damage to the plate which ordinarily should be very smooth toproduce quality prints.

To require remedial measures such as tapping or air pressure to removegranular carrier beads from the plate after the development step wouldadd greatly to the complexity of xerographic apparatus.

In addition it is found that fine chips, powder, filaments and dustwhich adhere to the surface of both round and irregular carrier beadsmay cause unsightly deposition in non-charged background areas of theplate and no round, nonround sorting system has been found which alsosatisfactorily removes these fines from the bead surface.

In order to secure nearly perfectly round carrier particles devoid ofnonrounds including surface adhering fines, various sorting systems areavailable in the art which have not proved entirely satisfactory.

For example sorting systems relying on revolving discs and centrifugalforces to separate rounds from nonrounds are quick and simple butprovide a very rough degree of sorting and often fail to remove fineswhich adhere tenaciously to the surface of larger particles in thebatch. Other systems are available each of which is lacking in somefundamental aspect, for example, simplicity, cost, speed of operation ordegree of refinement as to render it not completely satisfactory forinclusion in a commercial manufacturing process.

It is therefore an object of this invention to provide a sorting systemto separate rounds from nonrounds which overcomes the above noteddisadvantages and satisfies the above noted wants.

It is a further object of this invention to provide a sorting system ofgeneral usefulness and particularly use ful in providing round carrierparticles for use in developers in the process of xerography.

It is a further object of this invention to provide a sorting systemwhich is simple and inexpensive to construct and operate.

It is a further object of this invention to provide a sorting systemreadily adaptable for batch or continuous type operation.

It is a still further object of this invention to provide a round,nonround sorting system capable of a high degree of refinement for eachcycle of operation.

It is a still further object of this invention to provide a round,nonround sorting system capable of simultaneously removing largerirregulars as well as fine chip, filament, powder and dust particleswhich otherwise might adhere tenaciously to the periphery of largerparticles.

It is a still further object of this invention to provide a sortingsystem capable of separating rounds from nonrounds of the same material.

The foregoing objects and others are accomplished in accordance withthis invention by causing unsorted particles to be advanced relative toa conveyor so that particles are electrostatically attracted towards theconveyor surface as they are advanced relative thereto whereby roundsare caused to advance at a faster rate in response to advancing forcesand nonrounds being effected to a greater degree by frictional andelectrostatic retarding forces are caused to move at a slower rate oractually stick to the conveyor surface, thereby effecting the separationof rounds from nonrounds.

In one aspect of the invention merely advancing and moving the unsortedparticles relative to the conveyor surface may in and of itself besufiicient to triboelectrically generate electrostatic charges on theparticles or the surface of the conveyor or both to cause anelectrostatic attraction between the conveyor and the particles to aidin sorting rounds from nonrounds. This type of contact electrificationis called triboelectrification from the Greek tribein meaning to rub.

In another aspect of the invention the surface of the conveyor or theparticles or both may be charged from an external charge source. Thischarging may be accomplished both prior to and during processing and ifboth the particles and the conveyor surface are charged the charges willusually be of opposite polarities to increase the electrostatic force ofattraction between conveyor and particles. It will be understood that inthis aspect of the invention there also may be triboelectricallygenerated charges in addition to charges placed from external chargesources.

In both aspects of this invention it is thought that sorting of roundsfrom nonrounds is accomplished by the ability of the rounds in responseto advancing and moving forces, to overcome conveyor surface frictionand electrostatic forces to advance along the conveyor for example to around collection point while nonrounds including fines, under thecombined greater influence of conveyor surface friction andelectrostatic forces advance more slowly than rounds or are stoppedaltogether to remain on the conveyor surface where they may be conveyedfor example to a nonround collection point to be removed by any suitablemeans. The system herein is found to be preferred for separatingparticles having an average diameter of less than about 3,000 micronsand especially preferred for separating particles having an averagediameter of less than about 700 microns and greater than about 50microns.

Also, uniquely, in both aspects of the invention fine chips, filaments,powder and dust are removed from the surface of rounds to render themclean for commercial use.

The advantages of this novel sorting system will become apparent uponconsideration of the following detailed disclosure of the inventionespecially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is an isometric view of an apparatus embodiment for carrying outthe invention.

FIG. 2 is a partially schematic representation of another apparatusembodiment for carrying out the invention.

FIG. 3 is a partially schematic representation of yet another apparatusembodiment for carrying out the invention.

FIGS. 4A-4D are outline drawings of 17.5 X photographs of 4A a batch ofunsorted glass carrie'r core particles the rounds of an average diameterof about 600 microns, 4B nonrounds from said batch deposited after 1cycle according to an embodiment hereof; 4C nonrounds deposited after asecond cycle according to an embodiment hereof and 4D sorted roundsafter two processing cycles.

Referring now to FIG. 1 there is shown an apparatus embodiment for batchprocessing unsorted particle's according to the invention.

According to the first aspect of the invention unsorted particles 15 arestored in feeder 11, the rate of flow of particles onto the conveyor 16in the form of a fiat bottomed chute being regulated by gate 13.

Conveyor 16 is secured to vibrator 14 comprising flexible leaf springs17, reciprocating member 9, electromagnetic drive mechanism 19 connectedelectrically to electric controller 20 used to control the amplitude andfrequency of the vibration, the controller in turn connectedelectrically to electrical power source 21. Vibrator 14 and feeder 11are in turn secured to support frame 12.

The magnet in mechanism 19, energized by pulsating current, pulls theconveyor down and back toward mechanism 19, then the leaf springs 17return it up and forward to its original position, the direction oftravel of the conveyor determined by the angle of the leaf springs.

Although conveyor 16 is illustrated to be a fiat bottom type, which ispreferred in order to provide conveyor contact with a maximum number ofparticles, various other shaped conveyors may be used includingV-shaped, half round, tubular and radius bottom conveyors.

The particles are caused to advance down the tilted conveyor bygravitational forces and optionally by vibrational or other forces whichfor selected particle material and conveyor surface materialcombinations causes triboelectrically generated electrostatic charges tobe formed on particles and the conveyor surface to cause anelectrostatic attraction between same.

The rounds being to a lesser degree effected by frictional andelectrostatic retarding forces between themselves and the conveyorsurface gravitate toward round collection point 18 while nonroundsincluding fines are caused by the same retarding forces to remain on thesurface of the conveyor thus effecting sorting of rounds from nonrounds.

Of course the tilt angle, amplitude and frequency of vibration, ifvibrational motion is used, batch feed rate and other factors willdepend on many factors including each other, and the size and density ofthe particles to be sorted, the position in the triboelectric series ofthe unsorted particles and the conveyor surface material, the relativesoftness or hardness of the particles and the conveyor surface materialsand the degree of separation or sorting desired for each processingcycle.

It will be appreciated that the type of apparatus illustrated in FIG. 1is preferred for sorting a single batch of particles since the sortingprocess should preferably be interrupted periodically to wipe nonroundsand fines from the surface of the conveyor. Of course a particular batchof particles may be reprocessed a number of times depending upon thedegree of refinement which is desired. In addition with certaincombinations of materials an undesirably large charge may be built uponon the conveyor surface after periods of continuous use to cause evenrounds to stick to the conveyor. In these situations periodicdischarging of the conveyor for example during 1cleaning will reduce theelectric fields to a more desirable evel.

Vibration of the conveyor surface has been found to be especiallyadvantageous for conveyor surfaces which are relatively soft for examplethe conveyor surface used in Example I, which appears below, as opposedto the relatively harder amorphous selenium conveyor surface utilized inExample V and for shorter length conveyors since vibration increasesboth the sliding and the rolling contact of the particles with thesurface of the conveyor thus increasing triboelectric charging andtriboelectric attraction between the particles and the conveyor surfacefor a given conveyor length. Vibration is also found to be advantageousin those cases where the particle material and the conveyor surfacematerial are relatively far apart in the triboelectric series which whencoupled with low tilt angles and relatively low density particlematerial and small particles may cause, because of the magnitude of VanDer Walls and electrostatic forces, a particles including rounds tobecome fixed to the conveyor surface unless vibratory motion is used toprevent such sticking. The characteristics of the vibratory motion usedto sort a particular batch of material according to an embodiment ofthis invention may be widely varied to optimize sorting of rounds fromnonrounds. For example the amplitude of vibration of the vibratoryfeeder used in Example I and illustrated in FIG. 1 may be varied fromabout 20 mils to about 55 mils. Also in any specific application,frequencies can be varied as desired as well as the angle of the line ofamplitude.

It should be appreciated that if the angle of the line of amplitude isacute with respect to the upwardly extending portions of a tiltedconveyor surface at least the larger nonrounds may be caused to actuallymove up the conveyor surface to a round collection point while at leastthe larger nonrounds move down to provide a self cleaning featureespecially useful for sorting batches with a relatively low amount offines. It should be appreciated that in all embodiments of the presentinvention the conveyor need not be tilted since vibrating motion andmany other advancing forces may advance particles along for example ahorizontal conveyor.

In the first aspect of the invention, wherein the unsorted particlesthemselves triboelectrically generate charges by rolling or skidding incontact with the conveyor, it seems that at least the surface materialof the conveyor and of the particles must be selected in accordance withtheir tribolelectric properties so that when particles are advancedrelative to the surface of the conveyor either the particles or theconveyor surface is charged positively if the other material is below itin the triboelectric series and negatively if the other material isabove it in the triboelectric series to create sufficient triboelectriccharging to cause an appreciable separation of rounds from nonrounds foreach processing cycle.

For unsorted particles of a particular material or at least surfacecoated with a particular material, the material selected for the surfaceof the conveyor may be selected from a large group of materials thathave been tested and which occupy recognized positions in thetriboelectric series so that charges are generated upon advancement ofthe particles over such a surface. The magnitude of triboelectriccharging and the electrostatic forces generated thereby when twomaterials are contacted is roughly correlated with the degree ofseparation of the two materials in the triboelectric series.

There is still much to be learned about the triboelectric effects andthe relative position of the various materials in the triboelectricseries but it is generally found to be preferred herein for both aspectsof the invention, that at least the surface of either the conveyor orthe particles to be sorted should have a bulk electrical resistivitygreater than about ohm-centimeters.

All of the various published triboelectric series are not in completeagreement because of the many intangibles found to be a part of researchdone in this area.

But by consulting the various published series a reasonably accurateprediction can be made as to how a particular conveyor surface materialwill triboelectrically react with a particular batch of unsortedparticles.

Three of the most famous of such series are given below. Table I appearsin an article by Shaw and Leavy, E. W. L. Proc. of Royal Society, 138,p. 506 (1932) which gives a triboelectric series of a number of metalsas compared with silica. It will be noted] that selenium lies at thevery bottom of the series indicating that to sort silica or glass typeparticles according to this invention a selenium coated conveyor mayprobably be advantageously used in the first aspect of this invention tocreate appreciable triboelectric charging. I

Table II is one of the earliest of such series called the Smithsonianseries and may be found published in Smithsonian Physical Tables, p.375, 9th Rev. Ed. (1954).

Table III appears in an article by Hersh and Montgomery, TextileResearch Laboratories, 28, p. 903 (1956).

Lucite is a class of acrylic resins from Du Pont, Dacron is a synetheticfiber made from the condensation of dimethyl terephthalate and ethyleneglycol from Du Pont, Orlon is a synthetic fiber made principally frompolyacrylonitrile from Du Pont, Dynel is a synthetic fiber made bycopolymerization of 40% acrylonitrile and 60% vinyl chloride, Velon is aline of synthetic plastics and resins available from Firestone PlasticsCo., and Teflon is a tetrafluoroethylene polymer from Du Pont.

Other series are known to those skilled in the art and may be used andcompared with each. other to select a conveyor material which will mostprobably react triboelectrically with the particles to be sorted to beused in accordance with this invention to separate rounds fromnonroundsand to clean the surface of rounds of fine 1) Asbestos (sheet) (2)Rabbits fur (Hg) (3) Glass (combn. tubing) (4) Vitreous silica, opposumsfur (5) Glass (fusn.)

(6) Mica (8) Glass (pol.), quartz (pol.), glazed porcelain (9) Glass(broken edge), ivory (l0) Calcite (11) Cats fur (12) Ca, Mg, Pb,fluorspar, borax (13) Silk (14) Al, Mn, Zn, Cd, Cr, felt, hand,wash-leather (15) Filter paper 7 Vulcanized fiber Cotton MagnaliumK-alum, rock-salt, satin spar Woods, Fe Unglazed porcelain, salammoniacK-bichromate, parafiin, tinned Fe Cork, ebony (24) Amber (25) Slate,chrome-alum (26) Shellac, resin, sealing wax (27) Ebonite (28) Co, Ni,Sn, Cu, As, Bi, Sb, Ag, Pd, C, Te,

Eureka, straw, copper sulfate, brass (29) Para rubber, iron alum. (30)Guttapercha (31) Sulfur (32) Pt, Ag, Au (33) Celluloid (34) India rubberNegative TABLE III Positive Wool Nylon Viscose Cotton Silk AcetateLucite Polyvinyl alcohol Dacron Orlon Dynel Velon Polyethylene NegativeBecause most glasses have a bulk electrical resistivity greater thanabout ohm-centimeters it is found that for example when glass beads areto be sorted the conveyor may be surface coated with an electricallyconductive material including most metals as long as the electricallyconductive layer is backed with an electrical insulating layer which inturn is suitably backed with an electrically conductive layer, as Wellas any suitable electrically insulating material such as natural andsynthetic resins, and the like; so long as the material is separatedfrom the glass bead material in the triboelectric series.

On the other hand if the conveyor surface is of a material of bulkresistivity greater than about 10 ohm-centimeters particles of almostany material within the size ranges specified hereinabove may be sortedin accordance with this invention so long as the particle material or atleast its surface coating is suitably separated from the conveyorsurface material in the triboelectric series.

Any suitable film formable material of bulk resistivity greater thanabout 10 ohm-centimeters may be used as a conveyor surface coating.Typical such materials include polysulfone, polyethylene terephthalatepolyester, acrylics for example alphamethyl styrene copolymer, cellulosenitrate, epoxy resins, phenolics, phenolformaldehyde, silicones,urethanes, urea-formaldehyde, cellulose acetate, polycarbonates,cellophane, polychlorotrifluoroethylene copolymers, polyvinyl-butyral,polymethyl methacrylate, polystyrene, polyethylene, fiuorohalocarbons,cellulose triacetate, cellulose acetate butyrate, polyurethaneelastomer, cellulose propionate, ethyl cellulose, polypropylene,polyvinyl fluoride, vinyl-chloride-acetate copolymers, vinylidenechloride-vinyl chloride copolymer, tetrafiuoroethylene available underthe trademark Teflon from E. I. du Pont de Nemours & Co., copolymers ofhexafluoropropylene and polytetrafluoroethylene, polyvinyl chloride,

polyacrylonitrile, cellulose nitrate plasticized with camphor, hardrubber such as ebonite and chlorinated rubber, nylons or polyamides,polyvinyl alcohol, polyvinylidenefluoride, copolymers ofchlorotrifiuoroethylene and vinylidine fluoride, casein, polyglycols,alkyds and others.

In order to prevent dynamic charge build up resulting from extendedperiods of use of a sorting system of a conveyor surface and particlesrelatively far apart in the triboelectric series, certain film formablematerials which temporarily hold a charge are especially preferred foruse as conveyor surface materials herein since they may be charged tosort according to the invention but are electrically leaky or conductiveenough to continually discharge a part of the charge build up to preventovercharging and resultant sticking of even rounds to the conveyorsurface.

Preferred film formable materials which are electrically leaky for useherein include cellulose acetate, ethyl, cellulose, epoxy resins,phenolics, polyvinyl fluoride, polystyrenes and others.

It will be understood that various fillers, plasticizers and additivesmay also be added to the aforementioned film formable materials and toothers to change their electrical properties and that other suitableleaky materials are available in the art and may be used herein. In factit is generally found that any film formable material possessing adielectric constant and a bulk electrical resistivity such that theproduct of these tWo values is between about 10 and 10 ohm-centimetersis leaky and satisfies that requirement for use as a preferred conveyorsurface material herein.

In the second aspect of the invention, the conveyor surface or theparticles or both may be charged from an outside charge source ratherthan relying solely on triboelectric charging. This aspect of theinvention is found to be advantageous Where shorter conveyor lengths areto be used.

For maximum sorting effect the conveyor should be surface charged to anegative potential of the particles are of a material above the conveyorsurface material in the triboelectric series and to a positive potentialif the particles to be sorted are in a lower position in thetriboelectrieseries. For example to sort glass beads with a conveyorsurface of selenium which is below or negatively situated in thetriboelectric series in relation to the glass beads, the conveyorsurface should be charged negatively for maximum electrostaticattractive effect. If such a charge causes sticking of rounds to theconveyor, negative charging of the conveyor surface to a lower potentialor posi tive charging probably to a low potential might be tried inorder to lower the electrostatic attractive forces sufficiently topermit rounds to advance. Of course increasing the tilt angle of theconveyor and using vibratory motion are also useful in preventingsticking of rounds.

Photoconductive insulating materials such as amorphous selenium areadvantageously used as conveyor surface materials in this as well as thefirst aspect of the invention because in the absence of actinic lightsuch photoconductive insulating materials are electrically insulatingthus will accept and retain a charge applied either from an outsidecharge source or generated triboelectrically for example, during thesorting process. Then, for example by exposing the conveyor surfaceillustrated in FIG. 1 to actinic light and, for example, by increasingthe angle of the conveyor with the horizontal, nonrounds may be readilywiped from the conveyor surface, for example, by a vibratory or abrushing action, to prepare the apparatus for another batch to besorted. Also this periodic exposure discharges dynamic charge build up.

Any suitable hotoconductive insulating material may be used as aconveyor surface material. Typical ones include amorphous selenium,alloys of sulfur, arsenic or tellurium with selenium, selenium dopedwith materials such as thallium, cadmium sulfide, cadmium selenide,etc., particulate photoconductive materials such as zinc sulfide,

zinc cadmium sulfide, French process zinc oxide, phthalocyanine, cadmiumsulfide, cadmium selenide, zinc silicate, cadmium sulfoselenide, linearquinacridones, etc. dispersed in an insulating inorganic film formingbinder such as a glass or an insulating organic film forming binder suchas an epoxy resin, a silicone resin, an alkyd resin, a styrenebutadieneresin, a wax or the like. Other typical photoconductive insulatingmaterials include: blends, copolymer, terpolymers, etc. ofphotoconductors and non-photoconductive materials which are eithercopolymerizable or miscible together to form solid solutions and organicphotoconductive materials of this type include: anthracene,polyvinylanthracene, anthraquinone, oxidiazole derivatives such as2,5-bis-(p-amino-phenyl-l), 1,3,4-oxidiazole; 2- phenylbenzoxazole; andcharge transfer complexes made by complexing resins such aspolyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates,etc., with Lewis acid such as phthalic anhydride;2,4,7-trinintrofluorenone; metallic chlorides such as aluminum, zinc orferric chlorides; 4,4-bis(dimethylamino) benzophenone; chloranil; picricacid; 1,3,5-trinitrobenzene; l-chloroanthraquinone; bromal;4-nitrobenzaldehyde; 4-nitrophenol; acetic anhydride; maleic anhydride;boron trichloride; maleic acid; cinnamic acid, benzoic acid; tartaricacid; malonic acid and mixtures thereof.

The conveyor surface may be charged in a wide variety of Ways includingvigorously rubbing the surface with a softened material such as a cottonor silk handkerchief or a soft brush or fur chosen to impart charge ofthe desired polarity, induction charging for example, as described inWalkup Patent 2,934,649, roll charging as described in Straugham, Mayer,Proc. Nat. Electronics Conf. 13, 959-,- 962 (1958), depositing chargefrom a corona discharge device and other techniques. Charging by coronadischarge devices which generally can apply either positive or negativecharge of varying potentials and which may be adapted for manyapplications are found to be preferred outside source applicators foruse herein. For example corona discharge devices of the generaldescription and generally operated as disclosed in Vyverberg Patent2,836,725 and Walkup Patent 2,777,957 have been found to be excellentsources of corona useful in the charging of photoconductive insulatingmaterials or other conveyor surfaces. It will be understood that notonly may electrically insulating conveyor surfaces be charged inaccordance with the invention but electrical conductors, if properlyinsulated may also accept and retain a charge.

It is preferred in this second aspect of the invention if higherelectrostatic forces are being sought, that the conveyor surfacematerial and the particle material be selected according to their bulkelectrical resistivity and according to their relative placement in thetriboelectric series as described herein for use in the first aspect ofthe invention since triboelectrieally generated charges would add toexternally applied charges to increase the resultant electrostaticattraction. Of course for some materials and operating parameters suchselection is not necessary since an increase of the electrostaticattraction between the particles and the conveyor surface by reason oftriboelectrification would not be needed or desired. An advantage of thesecond aspect of the invention is that a conveyor surface electricallyconductive or insulating identical to the unsorted particle material maybe used to produce electrostatically generated forces between theconveyor surface and the particles according to the second aspect of theinvention.

Although uniform charging of the conveyor surface 1s found to createelectrostatic lines of force even in the middle of large charge areas,these lines of force may be strengthened for the same amount of chargepotentlal by utilizing various techniques such as development electrodesand screening techniques for example as described 1n Dessauer and Clark,Xerography, Focal Press, pp. 274-279 1965 In ad dition to charging theconveyor surface from an outside charge source the unsorted particlesmay be charged prior to their being advanced relative to the conveyorsurface by corona techniques or for example by utilizing a vibratingsupply hopper illustratively of the type available from Syntron Co.,Homer City, Pa., to cause particles held therein to acquire charge byrubbing against the sides of the feeder. The inside of the feeder maypreferably be coated with a layer of material selectively chosen fromthe triboelectric series to charge the particles to a polarity oppositeto the polarity of charge applied to the conveyor surface to enhance theeletcrostatic attraction between the conveyor surface and the particles.Such charging although not required may be desirable in certain systems.It will be understood in connection with the second aspect of theinvention that charging from external charge sources may take place notonly prior to the contact. of particles with the conveyor but during theadvancement of particles relative thereto.

The versatility of the sorting system disclosed herein is illustrated bythe various continuous or automatic apparatus embodiments for carryingout the invention two embodiments of which are illustrated in FIGS. 2and 3.

Referring now to FIG. 2, and according to a first aspect of theinvention, belt 22 carries unsorted particles 15 from a particlemanufacturing operation to deposit the particles in feeder 26. Gate 28allows a variable feed rate of particles 24 to conveyor 30 in the formof an endless belt. Conveyor 30 is held in tension by roller 32 anddrive roller 34 driven by motor 36 through drive means not shown toadvance conveyor 32 illustratively in a direction opposite to thedirection in which rounds will tend to gravitate.

The factors of tilt angle, vibrational characteristics, if vibration isused, size and density of particles to be sorted, the relative positionsin the triboelectric series of the conveyor surface and the particlematerial, the relative hardness of the conveyor surface and particlematerial and the degree of separation or sorting desired for eachsorting cycle are all interrelated and depend on each other in a beltconveyor configuration in much the same way as in the apparatusembodiment illustrated in FIG. 1.

The rate of advancement of the conveyor belt may be very low for exampleone inch per second or lower which is generally sufficient tocontinuously remove nonrounds and present clean conveyor areas to thecascading particles to effect significant sorting, especially when theparticles are fed at a preferred rate to create a monolayer i.e. a layerof particles one particle thick cascading down the conveyor. Theconveyor belt may be advanced at much higher rates such as 5 or 10inches per second or more especially when used in conjunction withgreater tilt angles and vibration.

Of course a monolayer of cascading particles is not required and greaterfeed rates may be used with an increase in the number of layers ofparticles advancing along the conveyor but the degree of sorting willcorrespondingly decrease because of less particle contact with theconveyor surface and because the greater particle flow forces tend toWash away nonrounds deposited on the conveyor surface, and moreprocessing cycles may be needed depending on the degree of refinementdesired.

As illustrated, with at least the surface of the conveyor and theparticle surface material being selectively chosen as herein described,the particles flow from gate 28, become engaged in rolling and slidingcontact with advancing conveyor 30 thereby triboelectrieally chargingboth the surface of the conveyor 30 and the particles. The rounds beingless effected than nonrounds by retarding forces tend to gravitatetoward collection point 18 while the advancement of nonrounds and finestoward collection point 18 is retarded more than for the rounds by bothfrictional forces and electrostatic forces. In fact the rate ofadvancement of nonrounds towards collection point 18 is stopped or atleast slowed to a velocity less than the velocity of the moving conveyorthus causing nonrounds to be carried away from round collection point 18and toward roller 32 and nonround collection point 38 whereat some ofthe larger nonrounds will be caused by gravitational and centrifugalforces to leave the surface of the conveyor. Nonrounds including fineswhich adhere tenaciously to the conveyor may be removed by rotatingbrush 40 powered by motor 42 through drive means not shown.

It will be understood that belt 30 may comprise a photoconductiveinsulator on a conductive substrate with sorting taking place in theabsence of actinic light. Also for example an exposure station or othercharge dissipating means may be positioned immediately subsequent torotating brush 40 in the direction of belt advancement to at leastpartially discharge the belt to prevent the build up of charge to anunworkable level.

Referring now to FIG. 3 there is illustrated an exemplary automatedapparatus embodiment for carrying out the second aspect of the inventionwhere the conveyor surface or the particles or both are charged from anexternal source and not solely by triboelectric means. Unsortedparticles may be fed to the conveyor in any convenient manner forexample similar to that manner illustrated in FIG. 2. In FIG. 3 conveyor44 illustratively comprises a surface coating of a photoconductiveinsulating material on a grounded electrically conductive substratewhich may be a thin film of a metal, for example, aluminum. Thephotoconductive insulating surface of the conveyor is charged by coronacharging device 52 illustratively shown to be emitting negative chargeproducing particles.

Preferably, conveyor 44 should be charged when the photoconductiveinsulating material is at its highest insulating value or when there isan absence of actinic radiation i.e. that electromagnetic radiation thatwould make the photoconductive insulating layer electrically conductive.To allow the charge to remain on the surface of the layer once depositedthere, charging and the balance of particle sorting should preferablytake place in the absence of that wavelength radiation or light to whichthe particular photoconductive material is sensitive. As particles aredischarged from gate 28 and begin their gravitational descent towardsround collection point 18, electrostatic forces generated between theconveyor surface and the particles together with frictional forces exerta selectively greater influence on nonrounds and fines thus slowingtheir rate of advancement to an average rate of advancement less thanthe rate of advancement of conveyor 44 or causing them to stick to theconveyor as is the case with most fines, thus causing nonrounds andfines to move towards roller 32 and nonround collection point 38. In theregion of roller 32 light source 54 aids in discharging the charge onthe photoconductive insulating material which will dissipate at leastsome of the electrostatic attraction that the conveyor holds fornonrounds and fines thus facilitating their removal. The largernonrounds by the co-action of gravitational and centrifugal forces willbe impelled into nonround collection station 38. Nonrounds includingfines which adhere to the surface of the conveyor may be brushed off bya brush similar to brushing means 40 illustrated in FIG. 2 or bybrushing means 56 illustrated in FIG. 3 which comprises a revolvingbrush 58, hood 60 and a vacuum flow line 62 connected to an externallypositioned vacuum source 63. A further description of brushing removalmeans 56 may be found in Walkup et al. Patent 2,832,977.

Of course for the belt conveyors illustrated in FIGS. 2 and 3 vibratorymotion may be imparted to the region of the conveyor surface betweenrollers 32 and 34 upon which the sorting is taking place by positioningvibratory means beneath the conveyor in said region or for example bycausing vibration of one or both of rollers 32 and 34.

The following examples further specifically define the present inventionwith respect to the use of frictional and electrostatically generatedretarding forces to sort rounds from nonrounds. The parts andpercentages are by weight unless otherwise indicated. The examples beloware intended to illustrate various preferred embodiments of the refiningsystem of this invention. Particles are fed in each example by releasingthem onto the conveyor with a minimum of momentum before contacting theconveyor.

Example I A steel conveyor chute about 20 inches in length, about 5inches wide with about 2 inch sides, shaped similar to the chuteillustrated in FIG. 1 and available as vibratory feeder Model F0l0 fromthe Syntron Co., Homer City, Pa. is coated with a coating solution ofabout 50 parts vinyl chloride-vinyl acetate copolymer available as EXON470 available from the Firestone Plastics Co., about 8 parts of LuxolFast Blue Dye available from the E. I. du Pont de Nemours & Co., andabout 200 parts methyl ethyl ketone, by placing a quantity of thecoating solution at the end of the conveyor chute and tilting theconveyor to about 30 from the horizontal allowing the coating solutionto run down the conveyor with any excess draining at the end. Theconveyor chute is air dried and tilted at an angle of about 10 with thehorizontal.

About a 10 1b. batch of unrefined glass beads having a density of aboutlbs. per cubic foot comprising rounds and nonrounds including fines, therounds of an average diameter of about 600 microns is fed uniformly at aconstant rate of about 2 /2 lbs. per minute onto and across the entirewidth of the conveyor at its elevated end and allowed to run down thechute while the chute is caused to vibrate at an amplitude of about 50mils at an angle of about 30 degrees with the horizontal at a frequencyof about 3,600 cycles/minute.

For purposes of illustration, FIG. 4A is a drawing of a 17.5 photographof rounds and nonrounds comprising the unrefined batch of particles.Fines are not illustrated.

Rounds advance down the conveyor chute and are collected in a receiverat the end thereof. A quantity of nonrounds including fines comprisingchips, filaments, powder and dust some of which is pulled by theconveyor surface from the surface of the rounds themselves are depositedon the coated conveyor chute. FIG. 4B is a drawing of a 17.5 photographof nonrounds deposited on the surface of the conveyor chute after afirst pass. After this first pass, the relatively more round materialwhich passed over the chute is taken from the receiver and passed asecond time over a cleaned conveyor chute in the same position andoperating under the same conditions to further refine the particles.FIG. 4C is a drawing of a 17.5 photograph of nonrounds includingdumbbells, angular particles and nonspherical curvilinear particleswhich remain behind on the conveyor chute after the second pass.

After the second pass a much refined batch of round materials iscollected in the receiver at the lower end of the conveyor. FIG. 4D is adrawing of a 17.5 photograph of the refined material after two passes asdescribed in this example.

Example II A coated conveyor as in Example I is tilted at an angle ofabout 30 degrees with the horizontal.

About a 10 lb. batch of the same type of particles as in Example I isfed uniformly at a constant rate of about 2 /2 pounds per minute acrossthe entire width of the conveyor at its elevated end and allowed to rundown the conveyor with no vibration of the conveyor.

With each processing cycle a significant amount of separation of roundsfrom nonrounds is found to occur.

Example III Example 11 is followed except that the conveyor chute issurface coated with a layer of a plastic, tetrafluoroethylene polymeravailable from E. I. du Pont de 13 Nemours & Co. under the trademarkTeflon and is tilted at angle of about 20 degrees with the horizontal.

A significant amount of separation of round from nonrounds is found tooccur for each processing cycle.

Example IV Example II is followed except that the conveyor surface islayered with about a mil layer of Mylar polyethylene terethalatepolyester film available from the E. I. du Pont de Nemours & Co.

A significant amount of sorting of rounds from nonrounds is found tooccur for each processing cycle.

Example V A removable bottom plate of a conveyor chute of the type used.in Example I, comprising about a 50 micron layer of amorphous seleniumon a substrate of aluminum of about 60 microns in thickness is uniformlycharged negatively by a corona discharge device available commerciallyas a part of the Model D Processor available from Xerox Corporation, toa substantially uniform surface potential of about 600 volts and removedfrom the Processor being careful to avoid exposure to room light andsecured to the bottom of the conveyor chute. Working in infra red lightis found to be satisfactory. The conveyor chute is placed at an angle ofabout 10 degrees with the horizontal.

About a 10 pound batch of the same type of unrefined material as inExample I, in the absence of room light, is fed uniformly at a constantrate of about 2 /2 pounds per minute across the entire width of theconveyor at its elevated end and allowed to gravitate down the nega-'tively charged selenium coated conveyor with no vibration of theconveyor.

A comparison of the refined material with the unrefined material beforeprocessng, indicates a significant increase of rounds and a decrease ofnonrounds in the refined material.

The conveyor chute is cleaned of nonrounds including fines to ready theplate for another batch to be processed by exposing the plate to roomlight thereby at least partially discharging the plate, increasing theangle of the plate with the horizontal and brushing with a brush made upof New Zealand sheared and dyed rabbit fur.

Example VI A conveyor chute of the type used in Example I is coated withan organic photoconductive coating solution of the photoconductor2,5-bis (p-aminophenyl)-1,3,4-oxadiazole available under the trademarkTO 1920 from Kalle & Co. and the resinous binder material Vinylite VYNS,a copolymer of vinyl chloride and vinyl acetate available from Carbideand Carbon Chemicals Co in diethylketone in proportion of about 30 gramsof the photoconductor and about 30 grams of the VYNS to about every 300milliliters of diethylketone. The solution is applied to the conveyorusing a gravure roller. The photoconductive solution air dries to athickness of about 10 microns.

The bottom of the conveyor is then uniformly charged negatively by acorona discharge device to a substantially uniform surface potential ofabout -l50 volts in the absence of room light. The conveyor is thenplaced at an angle of about degrees with the horizontal.

A batch of particles is processed as in the last three paragraphs ofExample V.

Example VII The bottom of a conveyor chute of the type used in Example Iis first layered with about a 10 mil thick film of Mylar which isovercoated with about a 60 micron layer of copper completelyelectrically insulated from the rest of the conveyor by the Mylar layer.The conveyor is placed at an angle of about 10 degrees with thehorizontal.

About a 10 lb. batch of Teflon heads the rounds of an average diameterof about 400 microns is fed uniformly at a constant rate across thewidth of the conveyor at its elevated end and allowed to run down theconveyor with no vibration of the conveyor at a rate to create about amonolayer of beads cascading down the conveyor.

With each processing cycle a significant amount of separation of roundsfrom nonrounds is found to occur.

Example VIII Example I is followed except that about a 10 lb. batch ofcopper beads the rounds of an average diameter of about 300 microns isfed uniformly at a constant rate across the width of the conveyor at itselevated end and allowed to run down the conveyor with the conveyorvibrating, the feed rate sufiicient to create about a monolayer of beadscascading down the conveyor.

With each processing cycle a significant amount of separation of roundsfrom nonrounds is found to occur.

Although specific components and proportions have been stated in theabove description of exemplary preferred embodiments of the sortingsystem hereof, other suitable materials as specified herein may be usedwith similar results. In addition, other materials may be added to themixtures of the materials specified herein or variations may be made inthe various processing steps or in the apparatus embodiments tosynergize, enhance or otherwise modify the properties of the sortingsystem hereof.

For example various methods are available to discharge the particles andthe conveyor surface to facilitate removal of nonrounds from theconveyor surface after sorting including using a fine aqueous mist orionized air to serve as a charge path to ground.

Also, although gravitational and vibrational forces are convenient foradvancing particles relative to the conveyor other advancing forces areavailable and any means which causes advancement of particles relativeto a conveyor surface may be suitable. For example, particles may besorted according to this invention by lightly brushing particles with acharged or uncharged brush along on a conveyor which may be tilted or ina horizontal position. Also gas jets for example directing a stream ofair may be used to advance particles.

It will be understood that various other changes in the details,material steps and arrangements of parts which have been hereindescribed and illustrated in order to explain the nature of theinvention will occur to and may be made by those skilled in the art upona reading of this disclosure, and such changes are intended to beincluded within the principle and scope of this invention.

What is claimed is:

1. A process for sorting an unsorted mixture of particles into roundsand nonrounds, wherein the rounds of said unsorted mixture have anaverage diameter less than about 3,000 microns, by imparting advancingforces to the unsorted particles tending to move the particles relativeto a conveyor surface wherein said conveyor surface is at least surfacelayered With a first material and the particles are at least surfacecoated with a second material, the first and second materials beingseparated in the triboelectric series at least one of said first andsecond materials having a bulk electrical resistivity greater than about10 ohm-cm; the process comprising, the steps of:

(a) surface charging said conveyor surface to a first polarity;

(b) charging at least some of the unsorted particles to a surface chargeof a second polarity, opposite in sign to said first polarity, prior tothe imparting of advancing forces to said particles; and

(c) imparting advancing forces to the unsorted particles tending to movethe particles relative to said conveyor surface wherein at least some:of said particles are advanced in rolling and sliding contact with said15 conveyor surface to additionally triboelectrically charge saidparticles to a second polarity and additionally triboelectrically chargethe conveyor surface to a first polarity.

2. A process according to claim 1 wherein the first polarity charge iselectrically positive or negative depending upon if the first materialoccupies a higher or lower position, respectively, relative to thesecond material in the triboelectric series and the second polaritycharge is electrically opposite in sign to said first polarity charge.

3. A process according to claim 2 wherein said conveyor surface issurface layered with a photoconductor in electrical contact With anelectrically conductive substrate.

4. A process according to claim 2 wherein said conveyor surface issurface layered with a film forming material wherein the product of thedielectric constant and the bulk electrical resistivity of said filmforming material is between about and 10 ohm-cm.

5. A process according to claim 1 wherein the rounds of said unsortedmixture have an average diameter between about 50 and 700 microns.

6. A process according to claim 1 wherein said conveyor surface issurface layered with a photoconductor in electrical contact with anelectrically conductive substrate.

7. A process according to claim 1 wherein said conveyor surface issurface layered with a film forming material wherein the product of thedielectric constant and the bulk electrical resistivity of said filmforming material is between about 10 and 10 ohm-cm.

8. A process for sorting an unsorted mixture of particles into roundsand nonrounds by imparting advancing forces to the unsorted particlestending to move the particles relative to a conveyor surface whereinsaid conveyor surface is surface layered with a photoconductor inelectrical contact with an electrically conductive substrate, theprocess comprising the steps of:

(a) imparting advancing forces to the unsorted particles tending to movethe particles relative to said photoconductor conveyor surface;

(b) providing an electrostatic attractive force between the unsortedparticles and the conveyor surface, thereby electrostaticallyattracting, at least during part of the advancing step, chargedparticles to the oppositely charged conveyor surface; and

(c) exposing said photoconductor to actinic light to at least partiallydischarge said photoconductor; whereby electrostatic and frictionalforces between said conveyor surface and said particles exert aselectively greater retarding influence on nonrounds causing rounds toadvance faster and further along the conveyor surface in response toadvancing forces as compared to nonrounds; to cause sorting of roundsand nonrounds.

9. Apparatus for sorting an unsorted mixture of particles into roundsand nonrounds, wherein the rounds of said unsorted mixture have anaverage diameter less than about 3,000 microns, comprising incombination:

(a) a flexible, elongate conveyor surface comprising a photoconductoroverlying an electrically conductive substrate, capable of accepting andretaining, at least temporarily, a uniform electrostatic charge;

(b) means for supporting and advancing said conveyor surface through apredetermined path;

(c) electrostatic charging means positioned adjacent said path adaptedto uniformly charge said conveyor surface as portionsof said surfacepass by;

(d) a particle feeding means next in the path of said conveyor surface,in the direction of its advancement positioned adjacent said pathadapted to feed particles to said conveyor surface;

(e) a conveyor surface cleaning means next in the path of said conveyorsurface positioned adjacent said surface and adapted to clean nonroundsincluding fine chips, filaments, powder and dust from said surface;whereby electrostatic and frictional forces between said conveyorsurface and said particles exert a selectively greater'retardinginfluence on nonrounds causing rounds to advance faster and furtheralong the conveyor surface in response to advancing forces, as comparedto nonrounds, to cause sorting of rounds from nonrounds.

10. Apparatus for sorting an unsorted mixture of particles into roundsand nonrounds, wherein the rounds of said unsorted mixture have anaverage diameter less than about 3,000 microns, wherein said particlesare at least surface coated with a first material and wherein saidconveyor surface is at least surface layered with a second material thefirst and second materials being separated in the triboelectric seriesat least one of said first and second materials having a bulk electricalresistivity greater than about 10 ohm-centimeters comprising incombination:

(a) a flexible elongate conveyor surface comprising a photoconductoroverlying an electrically conductive substrate, capable of accepting andretaining, at least temporarily, a uniform electrostatic charge;

(b) means for supporting and advancing said conveyor surface through apredetermined path;

(c) electrostatic charging means positioned adjacent said path adaptedto uniformly charge said conveyor surface as portions of said surfacepass by;

(d) a particle feeding means next in the path of said conveyor surface,in the direction of its advancement positioned adjacent said pathadapted to feed particles to said conveyor surface;

(e) a conveyor surface cleaning means next in the path of said conveyorsurface positioned adjacent said surface and adapted to clean nonroundsincluding fine chips, filaments, powder and dust from said surface;whereby electrostatic and frictional forces between said conveyorsurface and said particles exert a selectively greater retardinginfluence on nonrounds causing rounds to advance faster and furtheralong the conveyor surface in response to advancing forces, as comparedto nonrounds, to cause sorting of rounds from nonrounds.

11. Apparatus according to claim 10 including in combination a conveyorsurface charge dissipating means positioned adjacent said conveyorsurface and between said particle feeding means and said conveyorsurface cleaning means in the path of said conveyor surface taken in thedirection of advancement thereof.

12. Apparatus according to claim 11 wherein said charge dissipatingmeans is positioned between said particle feeding means and saidconveyor surface cleaning means.

13. Apparatus according to claim 11 wherein said charge dissipatingmeans comprises a source of actinic radiation for said photoconductor.

14. Apparatus for sorting an unsorted mixture of particles into roundsand nonrounds, wherein the rounds of said unsorted mixture have anaverage diameter less than about 3,000 microns, comprising incombination:

(a) a flexible, elongate conveyor surface comprising a film formingmaterial wherein the product of the dielectric constant and the bulkelectrical resistivity of said film forming material is between about 10and 10 ohm-centimeters;

(b) means for supporting and advancing said conveyor surface through apredetermined path;

(0) electrostatic charging means positioned adjacent said path adaptedto uniformly charge said conveyor surface as portions of said surfacepass by;

(d) a particle feeding means next in the path of said conveyor surface,in the direction of its advancement positioned adjacent said pathadapted to feed particles to said conveyor surface;

(e) a conveyor surface cleaning means next in the path of said conveyorsurface positioned adjacent said surface and adapted to clean nonroundsincluding fine chips, filaments, powder and dust from said surface;

whereby electrostatic and frictional forces between said conveyorsurface and said particles exert a selectively greater retardinginfluence on nonrounds causing rounds to advance faster and furtheralong the conveyor surface in response to advancing forces, as comparedto nonrounds, to cause sorting of rounds from nonrounds. I;

15. Apparatus for sorting an unsorted mixture of particles into roundsand nonrounds, ,wherein the rounds of said unsorted mixture have anaverage diameter of less than about 3,000 microns, by impartingadvancing forces to the unsorted particles tending to move the particlesrelative to a conveyor surface the, appaartus comprising in combination:

(a) a conveyor surface, surface layered with a photoconductor inelectrical contact with an electrically conductive substrate;

(b) means for imparting advancing forces to the unsorted particlestending to move the particles relative to said photoconductive conveyorsurface;

(c) means for providing an electrostatic attractive force between theunsorted particles and the conveyor surface, thereby electrostaticallyattracting, at least during part of the advancing step, chargedparticles to the oppositely charged conveyor. surface; and

(d) means to expose at least portions of said photoconductor toradiation to at least partially discharge electrostatic chargetherefrom; whereby electrostatic and frictional forces between saidconveyor surface and said particles exert a selectively greaterretarding influence on nonrounds causing rounds to advance faster andfurther along the conveyor surface in response to advancing forces ascompared to nonrounds; to cause sorting of rounds and nonrounds.

16. Apparatus for sorting an unsorted mixture of particles into roundsand nonrounds, wherein the rounds of said unsorted mixture have anaverage diameter of less than about 3,000 microns, by impartingadvancing forces to the unsorted particles tending to move the particlesrelative to a conveyor surface the apparatus comprising in combination:

(a) a conveyor surface, surface layered with a film forming materialwherein the product of the dielectric constant and the bulk electricalresistivity of said film forming material is between about 10' and 10ohm-cm;

(b) means for imparting advancing forces to the unsorted particlestending to move the particles relative to said conveyor surface; and

(c) means for providing an electrostatic attractive force between theunsorted particles and the conveyor surface, thereby electrostaticallyattracting, at least during part of the advancing step, chargedparticles to the oppositely charged conveyor surface; wherebyelectrostatic and frictional forces between said conveyor surface andsaid particles exert a selectively greater retarding influence onnonrounds causing rounds to advance faster and further along theconveyor surface in response to advancing forces as compared tononrounds; to cause sorting of rounds and nonrounds.

References Cited UNITED STATES PATENTS 714,649 11/1902 Sutton 209-1,744,967 1/ 1930 Johnson 209-131 2,314,939 3/ 1943 Hewitt 209-1273,059,772 10/1962 Le Baron 209-127 3,249,225 5/1966 Stuetzer 209-1292,638,416 5/1953 Walkup 252-621 OTHER REFERENCES Ralston, ElectrostaticSeparation of Mixed Granular Solids, Elsevier Pub. Co., N.Y., 1961, (TP156, E5R3), pages 30-36.

Fraas, Contact Potential in Electrostatic Separation, U.S.B.M. R1. 3667,1942,, pages 49.

Fraas, Electrostatic Separation of Granular Materials, U.S.B.M. Bull.603 (TN23U4), 1964, pages 12 and 13.

FRANK W. LUTTER, Primary Examiner

